The present invention relates to an apparatus for manufacturing an optical waveguide preform and more particularly, to an apparatus for maintaining the diameter of a preform bait tube during the deposition of glass layers therein.
Vapor deposition process have been commonly employed in the formation of optical waveguide preforms. In one such process, one or more layers of glass are formed on the inner surface of a glass bait tube by chemical vapor deposition or by other known techniques. A vapor phase reaction mixture is caused to flow through the bore of a glass bait tube while heating means such as a gas burner moves longitudinally along the tube to form a moving hot zone within the tube. The reaction mixture reacts in the hot zone to form a reaction product often referred to as "soot" which flows downstream from the hot zone where at least a portion thereof deposits on the inner surface of the tube where it becomes sintered to form a glassy layer. Ordinarily, the coated bait tube has at least two compositional regions. The interior region will ultimately form the core of the resultant optical fiber, and the exterior region will form the cladding thereof. The remaining critical step involves drawing the relatively large diameter cylindrical preform into a small diameter fiber. Prior to drawing the preform into a fiber, the preform is usually collapsed into a smaller diameter preform or preferably into a solid cylindrical mass.
Fibers having out-of-round cores and fibers wherein the core is not concentric with the outer cladding surface incur inordinately high splice losses during the coupling of such fibers. Also, the launching of radiation into an optical fiber and the propagation of the radiation therethrough can be adversely affected by fibers having nonuniform geometries. An optical fiber having the desired geometrical properties of circularity and concentricity can only be obtained from a preform having the same geometry. However, even when a circular bait tube is employed, the resultant preform may posses nonuniformities introduced during the deposition process. With each glass deposition pass of the heating means, the bait tube shrinks by a small amount due to surface tension, and when a burner is employed, the burner gas forces add to the tube shrinkage. Cumulatively, this effect can deform the bait tube geometry, thereby causing it to become out-of-round.